| The frequency of terahertz wave is between the microwave frequency and the frequency of far-infrared light.In recent years,it has attracted numerous attentions from many research groups due to its excellent performance in imaging and communication.Polarization plays a very important role in the transmission of electromagnetic waves.Polarization conversion devices are required in both polarization matching in the transmission and the mismatch of polarization during the transmission of electromagnetic waves to avoid interference.Conventional polarization conversion devices have problems such as narrow operating bandwidth,low conversion efficiency,large insertion loss and difficulty in integration in the terahertz frequency band.The negative dielectric constant,negative permeability and negative refractive index provided by the artificial electromagnetic metasurface can solve the above problems well.By attaching different metal patterns on the upper and lower layers of different dielectrics,the wavefront of the transmitted electromagnetic wave can be freely controlled.Therefore,in this thesis,we have designed a variety of reflective terahertz polarization conversion devices with the flexible material polyimide film based on the study of artificial electromagnetic metasurface,achieving high-efficiency broadband polarization conversion.The main research contents of this paper are as follows:1.We propose an efficient linear polarization converter of broadband terahertz wave in reflective mode.By adding a pair of resonance branches at both ends,the resonance frequency point in the band is increased and the operating bandwidth of the polarization converter is improved.The simulation results show that the proposed structure can convert a normally incident linearly polarized wave into an orthogonally polarized wave when the frequency band is about 0.42-0.65 THz with a polarization conversion rate greater than 0.99,which referred to a relative bandwidth of 43.0%.The physical mechanism of its high conversion efficiency is analyzed and explained by its surface current distribution and reflection phase.At the same time,the dependence of the structure on the incident angle of the terahertz wave is also studied.The results demonstrate that the structure can maintain good polarization conversion performance when the incident angle varies from 0° to 30°.2.Based on the high-efficiency broadband reflective linear polarization converter designed above,a terahertz reflective linear polarization converter with abnormal reflection is designed combined with the generalized Snell’s law and phase gradient metasurface theory.First,it explains how to realize abnormal reflection based on the generalized Snell’s law and phase gradient metasurface theory.We explained the abnormal reflection phenomenon through the combination of far field distribution and spatial electric field distribution.The simulation results show that the main lobe angles at0.40,0.45,0.50,0.55,and 0.60 THz are 39.0°,34.0°,30.0°,27.0°,and 25.0°,respectively,which are consistent with the calculation results.The side-lobe levels are all below-14 d B.Then the principle of high-efficiency broadband of each unit is explained.Because it can realize the abnormal reflection effect,the designed structure has a wider application prospect than the ordinary vertical reflection type linear polarization converter.3.In order to reduce the sidelobe level of the previous abnormal reflection type terahertz reflection type linear polarization converter and improve the transmission efficiency of directional reflection,a terahertz reflective linear polarization converter that can achieve off-axis focusing is designed based on the generalized Snell’s law and the field superposition effect.The designed polarization converter works at 0.5 THz.The focus length is about 30.0 mm in z-axis and the off-axis distance is about 4.0 mm.The simulation results show that the focusing effect is efficient at the predetermined focus,and the side lobes can be effectively reduced. |
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